1. Field of the Invention
The present invention relates to a power transfer device which is connected to AC power sources of different kinds, which normally supplies power from first AC power source to a load, and which, when an abnormality, such as a voltage drop, occurs in the power source, switches to another or second power AC power source so as to continuously supply power to the load.
2. Description of the Related Art
FIG. 12 is a diagram schematically showing the configuration of a conventional power transfer device which disclosed in, for example, U.S. Pat. No. 5,644,175, and FIG. 13 is a flowchart showing a procedure of a transferring operation in the conventional power transfer device shown in FIG. 12.
In FIG. 12, 1a denotes a first AC power source, 1b denotes a second AC power source, and 2 denotes a load which is connected to one of the AC power sources and requested to always operate.
The reference numerals 3a and 3b denote first and second current transfer switches (referred to also as current transferring section) which are connected between the first AC power source 1a and the load 2, and the second AC power source 1b and the load 2 to select an AC power source which supplies a current to the load 2, respectively.
The reference numerals 4a1 and 4a2 denote current directional semiconductor switches such as thyristors which constitute the first current transfer switch 3a, and which do not have self-arc extinguishing properties. The switches are connected in parallel so that their conduction directions are opposed to each other.
Similarly, 4b1 and 4b2 denote current directional semiconductor switches such as thyristors which constitute the second current transfer switch 3b, and which do not have self-arc extinguishing properties. The switches are connected in parallel so that their conduction directions are opposed to each other.
The reference numerals 5a1 and 5a2 denote gate drivers which supply gate signals to the semiconductor switches 4a1 and 4a2 constituting the first current transfer switch 3a, respectively, and 5bl and 5b2 denote gate drivers which supply gate signals to the semiconductor switches 4b1 and 4b2 constituting the second current transfer switch 3b, respectively.
The reference numerals 6a1, 6a2, 6b1, and 6b2 denote signal switches for switching over an ON signal (conduction signal) and an OFF signal (non-conduction signal) that are to be supplied to the gate drivers 5a1, 5a2, 5bl, and 5b2.
The reference numeral 7 denotes a voltage detector for detecting the voltage of the first AC power source 1a, 8 denotes a power interruption detector for detecting a power interruption of the first AC power source 1a on the basis of the voltage of the AC power source 1a which is detected by the voltage detector 7, and for outputting a power interruption signal, 9 denotes a current detector for detecting a current flowing from the first AC power source 1a to the load 2, and 10 denotes a current direction detector for receiving the power interruption signal from the power interruption detector 8, for detecting the current direction on the basis of the polarity of the current detected by the current detector 9 and flowing from the first AC power source 1a to the load 2, and for causing the signal switches 6b1 and 6b2 to select the ON signal or the OFF signal in accordance with a result of the detection.
The power interruption detector 8 causes the signal switches 6a1 and 6a2 to select the ON signal until a power interruption is detected (i.e., during a period when the first AC power source normally operates in a correct manner), and, when a power interruption is detected, outputs the power interruption signal (i.e., a power transfer signal for transferring from the first AC power source to the second AC power source) to cause the signal switches 6a1 and 6a2 to select the OFF signal.
Next, the operation of the conventional power transfer device will be described.
Referring to FIG. 12 showing the configuration of the conventional power transfer device, in a normal state, the signal switches 6a1 and 6a2 selects the ON signal, the gate drivers 5a1 and 5a2 supply the gate signal to the semiconductor switches 4a1 and 4a2 constituting the first current transfer switch 3a, the semiconductor switches 4a1 and 4a2 are therefore turned ON (enter the conduction state), and the first AC power source 1a is connected to the load 2, so that the power is supplied to the load from the first AC power source 1a. 
When an abnormality such as a power interruption occurs in the first AC power source 1a and the voltage applied to the load 2 is lowered, the power interruption detector 8 detects the power interruption on the basis of the voltage drop of the first AC power source 1a which is detected by the voltage detector 7, and generates the power interruption signal.
Upon reception of the power interruption signal output from the power interruption detector 8, the signal switches 6a1 and 6a2 select the OFF signal to cancel the gate signals for the semiconductor switches 4a1 and 4a2 constituting the first current transfer switch 3a. 
At this time, since the semiconductor switches 4a1 and 4a2 constituting the first current transfer switch 3a cannot perform self-arc extinguishing, the semiconductor switches 4a1 and 4a2 cannot enter the OFF state (non-conduction state) until the currents of the semiconductor switches 4a1 and 4a2 are reduced to a predetermined current level or lower, and hence the load 2 cannot be disconnected from the power source 1a. 
To comply with this, in the conventional power transfer device disclosed in U.S. Pat. No. 5,644,175, the current flowing from the first AC power source 1a is cancelled by a current flowing from the second AC power source 1b which is the sound AC voltage source, whereby the current flowing through the semiconductor switch 4a1 or 4a2 is lowered to the predetermined current level or less which is required for attaining the OFF state, so as to hasten the interrupting operation of the first current transfer switch 3a. 
Furthermore, at the timing when the first current transfer switch 3a is interrupted, the second current transfer switch 3b has been already turned ON, and hence the time period when the voltage applied to the load 2 is low can be shortened.
The operation procedure of the conventional power transfer device will be described with reference to the flowchart of FIG. 13.
In the following description, it is assumed that the direction of the current which is flowing from the first AC power source 1a to the load 2 at the occurrence of the power interruption coincides with that of the arrow shown above the current detector 9 in FIG. 12.
When a power interruption occurs in the first AC power source 1a (step 1 of FIG. 13) and the voltage of the first AC power source 1a detected by the voltage detector 7 is lowered, the power interruption detector 8 detects the power interruption (step 2 of FIG. 13), and then generates the power interruption signal.
In response to the power interruption signal generated by the power interruption detector 8, the signal switches 6a1 and 6a2 switch over from the ON signal to the OFF signal, and the gate signals generated by the gate drivers 5a1 and 5a2 are cancelled (step 3 of FIG. 13).
In response to the cancellation of the gate signals, the semiconductor switches 4a1 and 4a2 constituting the first current transfer switch 3a enter a state where the switches can be turned OFF at any time. Since the semiconductor switches 4a1 and 4a2 are semiconductor elements which cannot perform self-arc extinguishing, however, the semiconductor switch 4a1 through which the current is flowing continues to be in the ON state until the current is reduced to a fixed current level or less, although the semiconductor switch 4a2 having the current directionality which is opposite to the current flowing from the first AC power source 1a to the load 2 is immediately turned OFF.
After the gate signals of the gate drivers 5a1 and 5a2 are cancelled, the current direction detector 10 detects, for the second time, the current direction from the current detected by the voltage detector 7 (step 4 of FIG. 13). If the detected current directions detected both the first and second times coincide with each other, the current direction detector gives a signal to the signal switches 6b1 and 6b2 so that the gate drivers 5b1 and 5b2 generate gate signals which cause the semiconductor switch 4b1 of the second current transfer switch 3b to be turned OFF, and the semiconductor switch 4b2 to be turned ON, respectively.
When the gate drivers 5bl and 5b2 receive the signals from the signal switches 6b1 and 6b2, the gate drivers first cause only the semiconductor switch 4b2 to be turned ON (step 5 of FIG. 13). When the voltage of the second AC power source 1b is higher than that of the load 2, a current flows from the second AC power source 1b into the load 2. As a result, the current flowing from the first AC power source 1a into the load 2 is cancelled, so that the interruption of the semiconductor switch 4a1 (i.e., the turning OFF of the first current transfer switch 3a) can be hastened (step 6 of FIG. 13).
Thereafter, the other semiconductor switch 4b1 of the second current transfer switch 3b is turned ON (step 7 of FIG. 13), whereby the power transferring operation from the first AC power source 1a to the second AC power source 1b is completed (step 8 of FIG. 13).
In the operation, the current direction detector 10 detects the current direction twice, and, if the detected current directions detected both times coincide with each other, the current direction detector gives the ON and OFF signals to the semiconductor switches of the second current transfer switch 3b, because of the following reason. When the current direction is accidentally inverted after the current direction is once detected, the first current transfer switch 3a and the second current transfer switch 3b cause the first AC power source 1a and the second AC power source 1b to be short-circuited.
The semiconductor switch 4b2 only is turned ON because of the following reason. When the semiconductor switch 4b1 is turned ON under a state where the semiconductor switch 4a1 is turned ON, the first AC power source 1a and the second AC power source 1b are short-circuited.
As described above, when an abnormality such as a power interruption occurs in the first AC power source, the conventional power transfer device shown in FIG. 12 operates in the following manner to transfer from the first AC power source 1a to the second AC power source 1b. First, the gate signals for the semiconductor switches 4a1 and 4a2 constituting the first current transfer switch 3a are cancelled. The direction of the current flowing from the first AC power source 1a to the load 2 is detected twice. If the detected current directions coincide with each other, among the semiconductor switches constituting the second current transfer switch 3b, only the semiconductor switch of the polarity in which the current flowing from the first AC power source 1a to the load 2 is cancelled is turned ON, whereby the first AC power source 1a is rapidly interrupted while preventing the power sources from being short-circuited, so that the time period when the voltage applied to the load 2 is low can be shortened.
The conventional power transfer device is configured as described above, and has an advantage that the power sources are prevented from being short-circuited. However, the device has a problem that, when the current direction is accidentally inverted after the current direction detection, the semiconductor switch of the direction in which the first AC power source 1a and the second AC power source 2a are short-circuited is turned ON, thereby causing the first AC power source 1a and the second AC power source 2a to be short-circuited.
In the conventional power transfer device shown in FIG. 12, the first current transfer switch 3a which normally operates is configured by the semiconductor switches 4a1 and 4a2. However, the semiconductor switches generate a conduction loss, and hence the efficiency is poor, thereby increasing the running cost.
In order to dissipate the thermal energy due to the loss, a cooling structure is required, thereby producing a problem that the size of the device is increased.
It is an object of the invention to solve the above problems and to provide a power transfer device which, when a voltage abnormality is caused by occurrence of an abnormality such as power interruption in a first power source that normally operates, can rapidly transfer to a second power source while preventing the power sources from being short-circuited, and also a power transfer device in which the loss of a current transfer switch is small, and the efficiency is higher, and which can be miniaturized.
It is another object of the invention to provide a power transfer device which, even when a power transfer signal is generated by a manual operation or the like, can rapidly transfer to a second power source while preventing power sources from being short-circuited, and also a power transfer device in which the loss of a current transfer switch is small, and the efficiency is higher, and which can be miniaturized.
According to a aspect of the invention, there is provided a power transfer device of the invention comprises:
a first current transferring section, connected between a first AC power source and a load, for causing a current supply to the load to be in a conduction state or a non-conduction state;
a second current transferring section, connected between a second AC power source and the load, for causing a current supply to the load to be in a conduction state or a non-conduction state, the second current transferring section being configured by switches which have opposite conduction directions, respectively, which are connected in parallel, and which have first and second current directionalities, respectively;
a current detector for detecting a current which is supplied from the first AC power source to the load, and for outputting a detection signal;
a power transfer signal generator for generating a power transfer signal instructing transfer from the first AC power source to the second AC power source, the power transfer signal setting the first current transferring section to enter the non-conduction state;
a current direction estimator for, on the basis of output signals of the power transfer signal generator and the current detector, estimating a direction of a current which flows from the first AC power source to the load at a timing when the second current transferring section becomes conductive after an elapse of a preset time period; and
a conduction signal generator for, on the basis of a result of the estimation of the current direction estimator, first causing one of the switches constituting the second current transferring section and respectively having the first and second current directionalities to be conductive, a conduction direction of the one switch coinciding with a direction along which the current flowing from the first AC power source to the load is cancelled, and thereafter causing another one of the switches to be conductive.
The power transfer signal generator of the power transfer device of the invention is power source abnormality detector for detecting an abnormality of the first AC power source, and uses an output signal of the power source abnormality detector as the power transfer signal.
The power source abnormality detector of the power transfer device of the invention is power interruption detector for detecting a power interruption of the first AC power source.
The power transfer signal generator of the power transfer device of the invention generates the power transfer signal in response to a manual operation.
The first current transferring section of the power transfer device of the invention is a non-directional switch.
According to another aspect of the invention, there is provided a power transfer device of the invention comprises:
a first current transferring section which is connected between a first AC power source and a load, and which consists of a non-directional switch that causes a current supply to the load to be in a conduction state or a non-conduction state;
a second current transferring section, connected between a second AC power source and the load, for causing a current supply to the load to be in a conduction state or a non-conduction state, the second current transferring section being configured by switches which have opposite conduction directions, respectively, which are connected in parallel, and which have first and second current directionalities, respectively;
a current detector for detecting a value of a current which is supplied from the first AC power source to the load, and for outputting the current value;
a power transfer signal generator for generating a power transfer signal instructing transfer from the first AC power source to the second AC power source, the power transfer signal setting the first current transferring section to enter the non-conduction state; and
a current direction detector for, on the basis of the power transfer signal and an output signal of the current detector, detecting a direction of the current flowing from the first AC power source to the load, and for, on the basis of the detected current direction, first causing one of the switches constituting the second current transferring section and respectively having the first and second current directionalities to be conductive, a conduction direction of the one switch coinciding with a direction along which the current flowing from the first AC power source to the load is cancelled, and thereafter causing another one of the switches to be conductive.